High temperature tantalum base alloys



United States Patent HIGH TEMPERATURE TANTALUM BASE ALLOYS Rudolf H. Thielemann, Palo Alto, Calif, assignor t Sierra Metals Corporation, a corporation of Delaware No Drawing. Application July 1, 1957 Serial No. 668,891

6 Claims. (Cl. 75-174) This invention relates to a tantalum-chromium base metal 3, a d particularly to one such alloy whi h t;

(1) that it may be hot-worked, (2) is highly resistant to oxidation and other forms of corrosion at temperatures up to about 2000 F. and higher, and (3) possesses great mechanical work strength at these elevated temperatures. As a result this alloy may be used as blades, vanes, and other parts of. high temperature gas turbine engines. Other important uses'of the alloy of this invention are that it may be used as exhaust valves and manifolds in internal combustion engines, in heat exchangers, and in linings for retorts and, container vessels used in the N chemical and metallurgical industries.

Tantalum has a melting point of about 5430 F. an

at elevated temperatures (1. e., of the order of 2370 F.)

has greater strength properties than molybdenum under the same conditions.

Tantalum does not have the embrittling characteristics of molybdenum. Unlike molybdenum welds, tantalum welds are ductile and are not subject to cracking.

It is important to, note that substantially pure tantalum is, for all practical purposes, non-utilizable as a material for gas turbine blades, turbine vanes and turbine buckets, rocket nozzles and the like, because at temperatures of the order of 1500 F. and higher and in the presence of flowing air or corrosive gases for an extended period of time it oxidizes rapidly.

As to substantially pure tungsten, for all practical purposes it is non-utilizable because of the degree of difficulty in working the metal and the fact that it becomes embrittled when subjected to high temperature heating and cooling cycles.

The prior art high temperature, high strength base metal alloys such as the nickel and/or cobalt base metal alloys which have been used as blades, vanes, and other parts of high temperature gas turbine engines, have a maximum operating temperature of about 1500 F. For example, a common nickel-cobalt base metal alloy which incorporates molybdenum as a constituent is, for all practical purposes, non-utilizable as a structural member in a gas turbine engine if the metal temperature significantly exceeds about 1500 F. because of the strength and oxidation resistance limitations of such an alloy above this temperature.

The alloy of this invention when used as a blade or vane in a high temperature gas turbine engine can be operated at markedly higher temperatures than was possible heretofore. As a consequence, the performance of the gas turbine engine is improved in that the total thrust is increased and the amount of fuel consumed per pound of thrust per hour is decreased.

An alloy of the present invention is resistant to oxidation and has high work strength at elevated temperatures of the order of 2000 F. and higher, so as to be suitable invention for use in forming liners for retorts and container vessels used in the chemical and metallurgical industries.

The metal alloy of this invention is comprised by weight of approximately: 5 .percent to 20 percent chromium; 2 percent to 25 percent tungsten; and the balance, essentially tantalum. The proportions of the preferred alloys of the invention are approximately 10 percent to 20 percent chromium; 2 percent to 25 percent tungsten; and the balance, essentially tantalum.

It is important to note that in the past it has been the general understanding that the metals columbium and tantalum are subsantially equivalent. I have found that this is not the casein the present invention. Thus, for example, a relatively small proportion of chromium as an alloying constituent with columbium and tungsten renders the resulting columbium-chromium-tungsten metalalloy too brittle to be of any practical use, even when using amounts of tungsten up to 25 percent. On the other hand, I have discovered that up to 25 percent of tungsten and materially larger proportions of chromium can be alloyed with tantalum resulting in an alloy having unexpected, improved properties as indicated above. These properties are not achieved with a columbium-chromiumtungsten metal alloy having similar proportions.

To achieve the optimum desired properties in an alloy of the present invention, the impurities namedbelow prefverably should be held to the following approximate limits by weightinthe alloy. The carbon content in the final alloy preferably should be no more than 0.5 percent; the oxygen content, no more than 0.8 percent, as determined by an increase on ignition technique; the nitrogen content, no more than about 0.2 percent; and the iron content, no more than about 5 percent.

The following are examples of the preparation and test results of the tantalum-chromium base metal alloy of this Example 1 An ingot of a tantalum-chromium base metal alloy composition containing by weight 15 percent of chromium, 15 percent of tungsten, and the balance, essentially tantalum was prepared by are melting an electrode of tantalum to which 15 percent of chromium and 15 percent of tungsten were added. This may be accomplished by pressing a uniform powder mix of tantalum, chromium and tungsten in the above proportions in the form of bars under a pressure of about 50 tons per square inch, sintering the pressed bars under a vacuum condition of about one micron for a period of about three hours, and then arc melting the sintered bars under a vacuum condition of about 5 microns or less.

The are melted alloy of this example was tested for oxidation resistance in moving air at about 2000 F. for twenty-four hours and its resistance to oxidation is about 700 times greater than that of substantially pure tantalum.

The method employed for determining the degree of oxidation resistance consisted of preparing test samples of the alloy of this example and determining the dimensions of each test sample prior to subjecting it to the oxidation test conditions. The oxide film which formed on the surfaces of the test samples during testing was removed and the thickness of each tested sample was then measured and compared with the thickness of the test sample prior to subjecting it to testing. The same procedure was followed with substantially pure tantalum, and a comparison made.

Test bars inch diameter and 3 inches long) were fabricated from the arc melted ingot of this example by hot working procedure. The test bars had a -hour rupture strength in moving air which exceeded 20,000 pounds per square inch (p. s. i.) at about 2000 F.

3 Example 2 An ingot of a tantalum-chromium base metal alloy composition containing by weight 10 percent of chromium, 10 percent of tungsten, and the balance, essentially tantalum was prepared in accordance with the method set forth in Example 1.

The are melted alloy of this example was tested for oxidation resistance in moving air at about 2000 F. for twenty-four hours in the same manner as set forth in Example 1, and the test samples of this example had an oxidation resistance of about 500 times that of substantially pure tantalum tested under the same conditions.

Test bars inch diameter and 3 inches long) were fabricated from the arc melted ingot of this example by hot working procedure. The test bars of this example had a 100-hour rupture strength in excess of 20,000 p. s. i. at a temperature of about 2000 F. in moving air.

Example 3 resistance of about 100 times that of substantially pure tantalum tested under the same conditions.

Test bars A inch diameter and 3 inches long) were fabricated from the arc melted ingot of this example by hot working procedure. Test bars of this example had a 100-hour rupture strength in excess of 20,000 p. s. i. at about 2000 F. in moving air.

Example 4 An ingot of a tantalum chromium base metal alloy composition containing by weight 20 percent of chromium, 2 percent of tungsten, and the balance, essentially 4 tantalum was prepared in accordance with the method set forth in Example 1.

The are melted alloy of this example was tested for oxidation resistance in moving air at about 2000 F. for twenty-four hours in the same manner as described in Example 1. The oxidation resistance of the alloy of this example was about 800 times greater than that of substantially pure tantalum tested under the same conditions.

The test bars /4 inch diameter and 3 inches long) were fabricated from the arc melted ingot of this example by hot working procedure. Test bars of this example had a 100-hour rupture strength in excess of 20,000 p. s. i. at a temperature of about 2000 F. in moving air.

I claim:

1. A metal alloy which comprises by weight: percent to 20 percent chromium; 2 percent to 25 percent tungsten; and the balance, essentially tantalum.

2. A metal alloy which comprises by weight: percent to 20 percent chromium; 2 percent to 25 percent tungsten; and the balance, essentially tantalum.

3. A metal alloy which comprises by weight: percent chromium; 15 percent tungsten; and the balance, essentially tantalum.

4. An alloy which comprises by Weight: percent chromium; 2 percent tungsten; and the balance, essentially tantalum.

5. An alloy which comprises by weight: 5 percent to 20 percent chromium; 2 percent to percent tungsten; and the balance, essentially tantalum, the impurities, carbon, oxygen, nitrogen, and iron not exceeding about 0.5 percent of carbon, about 0.8 percent of oxygen, about 0.2 percent of nitrogen and about 5 percent of iron.

6. An alloy which comprises by Weight: 10 percent to 20 percent chromium; 2 percent to 25 percent tungsten; and the balance, essentially tantalum, the impurities, carbon, oxygen, nitrogen, and iron not exceeding about 0.5 percent of carbon, about 0.8 percent of oxygen, about 0.2 percent of nitrogen and about 5 percent of iron.

No references cited. 

5. AN ALLOY WHICH COMPRISES BY WEIGHT: 5 PERCENT TO 20 PERCENT CHROMIUM; 2 PERCENT TO 25 PERCENT TUNGSTEN; AND THE BALANCE, ESSENTIALLY TANTALUMM THE IMPURITIES, CARBON, OXYGEN, NITROGEN, AND IRON NOT EXCEEDING ABOUT 0.5 PERCENT OF CARBON, ABOUT 0.8 PERCENT OF OXYGEN, ABOUT 0.2 PERCENT OF NITROGEN AND ABOUT 5 PERCENT OF IRON. 